As a signal which is received by a base station apparatus is interfered by various signals, its reception quality is deteriorated. An array antenna is known as the technology that suppresses this interference and intensely receives electric waves to be received which come only from the direction of arrival. The array antenna can intensely receive electric waves to be received which come only from the direction of arrival by adjusting the amplitudes and phases which are to be given to reception signals by adjusting the weight coefficient (hereinafter called “weight”) by which the reception signals are to be multiplied.
Further, there is Rake reception that is a technique to enhance the reception quality. Rake reception can provide a path diversity gain by combining signals coming through different paths on the time axis even under a multipath environment. In general, even a base station apparatus equipped with an array antenna often performs this Rake reception.
In this case, the base station apparatus estimates the direction of arrival of a signal coming through each path and receives the signal with the directivity that is set in the estimated direction by the array antenna. The following will describe a conventional adaptive antenna base station apparatus which carries out both array reception and Rake reception.
FIG. 8 is a block diagram showing the structure of the conventional base station apparatus. In case where there are M paths, the base station apparatus comprises N antennae 60-1 to 60-N, N reception sections 61-1-1 to 61-N, N×M despreading sections 62-1-1 to 62-1-N, . . . , and 62-M-1 to 62-M-N, M beamforming sections 64-1 to 64-M, M direction-of-arrival (DOA) estimating sections 63-1 to 63-M, a Rake combining section 65 and a data determining section 66, where N and M are integers. Those components will be elaborated below.
In the conventional base station apparatus, signals which are to be received by the N antennae 60-1 to 60-N are subjected to radio processing (downconversion, A/D conversion, etc.) by the N reception sections 61-1-1 to 61-N which are provided in association with the antennae 60-1 to 60-N, and are then input to the N×M despreading sections 62-1-1 to 62-1-N, . . . , and 62-M-1 to 62-M-N.
The despreading sections 62-1-1 to 62-1-N, . . . , and 62-M-1 to 62-M-N perform a despreading process on signals which respectively come through the first path to the M-th path. That is, the despreading sections 62-p-1 to 62-p-N perform N despreading processes on the output signals of the N reception sections 61-1-1 to 61-N according to the reception timing for the signals that come through the p-th path (p being an integer from “1” to “M”). Accordingly, the despreading sections 62-p-1 to 62-p-N acquire signals on the p-th path (hereinafter called “path-p signals”) which have been received by the N antennae 60-1 to 60-N. N signals on the path p, which are the outputs of the despreading sections 62-p-1 to 62-p-N are input to the DOA estimating section 63-p and the beamforming section 64-p.
The DOA estimating sections 63-1 to 63-M respectively estimate the directions of arrival θ1 to θM of the path-1 to path-M signals. The estimated directions of arrival θ1 to θM are respectively input to the M beamforming sections 64-1 to 64-M.
The beamforming section 64-p combines N path-p signals or the output signals of the despreading sections by multiplying those signals by a reception weight, generated by using a direction of arrival (DOA) θp. Accordingly, the beamforming section 64-p outputs a single array-combined path-p signal and the beamforming sections 64-1 to 64-M output M array-combined signals which are to be input to the Rake combining section 65.
The Rake combining section 65 multiplies the input M array-combined path-p signals respectively by complex conjugates (S1)* to (SM)* of channel estimated values S1 to SM to compensate for channel variations h1, to hM, and then Rake-combines the signals. The Rake-combined signal is demodulated in the data determining section 66, thus yielding received data.
A description will now be given of a beamforming scheme as an operation of estimating the direction of arrival which is carried out in the conventional base station apparatus. The DOA estimating section 63-p forms a p-th path signal vector x(k), given by an equation 1 below, from a signal Xn(k) from the antenna 60-n at a sample time kΔT (where k is a natural number and ΔT is a sampling interval) of the p-th path signal, and computes a correlation matrix R, expressed by an equation 2, by using the p-th path signal vectors x(k) accumulated over a predetermined Ns sample period, where n=1 to N, T indicates transpose and H indicates complex conjugate transpose.x(k)=[X1(k)X2(k) . . . XN(k)]T  (1)
                    R        =                              1                          N              s                                ⁢                                    ∑                              k                =                1                            Ns                        ⁢                                          x                ⁡                                  (                  k                  )                                            ⁢                                                x                  ⁡                                      (                    k                    )                                                  H                                                                        (        2        )            
The angular spectrum is computed by changing θ of a DOA evaluation function F(θ), given by an equation 3, using the correlation matrix R obtained from the equation 2, and then the position of a maximum peak is detected as a direction-of-arrival (DOA) estimated value θp. It is to be noted however that a(θ) is a steering vector which is determined by the layout of the array antenna elements and can be expressed as an equation 4 in case of an equidistance linear array with an element interval d. In the equation 4, λ is the wavelength of a carrier and θ is an angle with the normal direction of the array set to be the direction of 0°.F(θ)=aH(θ)Ra(θ)  (3)
                              a          ⁡                      (            θ            )                          =                  [                                                    1                                                                                      exp                  ⁢                                      {                                                                  -                        j                                            ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              d                        ·                        1                        ·                        sin                                            ⁢                                                                                          ⁢                                              θ                        /                        λ                                                              }                                                                                                      ⋮                                                                                      exp                  ⁢                                      {                                                                  -                        j                                            ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              d                        ·                                                  (                                                      N                            -                            1                                                    )                                                ·                        sin                                            ⁢                                                                                          ⁢                                              θ                        /                        λ                                                              }                                                                                ]                                    (        4        )            
However, the conventional adaptive antenna base station apparatus with the above-described structure acquires the angular spectrum path by path and requires, for the respective paths, the DOA estimating sections which perform the same operation., This disadvantageously increases the scale of the apparatus. In case of Rake-combining M path signals, the apparatus should have M DOA estimating sections that perform the same operation. In case of acquiring the angular spectrum, the operations shown in the equations 3 and 4 should be performed. This raises another problem that the amount of computation which should be done by the conventional adaptive antenna base station apparatus increases exponentially in accordance with an increase in the number of the antenna elements and an increase in the number of the paths to be subjected to Rake combining. Further, as the structure is prepared for each communication terminal, if the number of communication terminals (i.e., the number of channels) with which the base station apparatus can communicate simultaneously increases, the scale of the apparatus and the amount of computation are further increased. With the recent significant leap in the quantity of communication terminal data, the scale of the apparatus and the amount of computation tend to increase further.